1. Field of the Invention
This invention relates to a control system for internal combustion engines for vehicles, which is capable of reducing a shock generated during a so-called power-on downshift of an automatic transmission of the engine, i.e. a downshift taking place when an accelerator pedal of the vehicle has been depressed.
2. Description of the Related Art
Conventionally, to reduce a shock generated during a power-on downshift of an automatic transmission of an automotive vehicle, there are known control systems for internal combustion engines for vehicles. One of such control systems changes hydraulic pressure supplied to a clutch of a target speed position (lower-speed clutch), based on the rotational speed of the engine assumed just before the downshift. Another control system instantaneously lowers the output torque of the engine by retarding the ignition timing of the engine at a timing when the downshift is almost completed. Still another control system delays the downshift-starting time point so as to start the downshift under predetermined conditions. These control systems are all intended to prevent an excessive increase in the engine rotational speed immediately after the downshift.
However, the above known control systems have the following disadvantages: That is, when a downshift is performed in a low vehicle speed region, the difference in engine rotational speed between before and after the downshift is so small that the downshift is completed in a very short time period (downshift duration), and therefore the hydraulic pressure supplied to the target speed position (lower-speed) clutch cannot sufficiently rise within the very short time period. In such a case, even if the hydraulic pressure supplied to the target speed position clutch is changed to a higher pressure or the ignition timing is retarded, a shock caused by the downshift cannot be reduced. Further, if the downshift-starting point is delayed until the hydraulic pressure supplied to the target speed position clutch rises, it can give a delayed, disjointed control; feeling to the driver.